1 and 3 are the same model. Calling a variable a nuisance factor is just
a description. The model is actually
y = age*meditation*b1 + meditation*b2 + err = y_hat + err
doug
On 8/13/13 6:12 PM, ye tian wrote:
Dear Freesurfers,
I am studying cortical thickness as a function of age (continuous) and
meditation practice (categorical, level1=Normal, level2=TM). I have
three separate cases of qdec, regression1, regression2 and regression3.
Regression1: Discrete (Fixed Factor) = meditation; Continuous
(Covariate) = age
Regression2: Discrete (Fixed Factor) = meditation;
Regression3: Discrete (Fixed Factor) = meditation; Nuisance Factor = age
For the question "Does the average thickness differ between Normal and
TM?", Regression1 and Regression3 have identical result, which is
different from Regression2, i.e., (1 = 3 !=2 ). May I infer from this
result the following about qdec:
1) Regression1 and Regression3 both fit cortical thickness,y, to the model
y = b0 + age*b1 + meditation*b2 + err = y_hat + err
Regression2 fits:
y = b0 + b2*meditation + err = y_hat + err
2) Some function of y_hat goes into the "average thickness", hence
the difference between 2 and the others.
However, what role does "Nuisance Factor" play in this case?
Thank you very much!
Sincerely,
Ye
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